1. Technical Field
The present disclosure relates generally to radio frequency (RF) signal circuitry, and more particularly, to wireless local area network (WLAN) power amplifiers with a co-existence filter for mobile wireless communications devices.
2. Related Art
Wireless communications systems find application in numerous contexts involving data transfer over long and short distances alike, and there exists a wide range of modalities suited to meet the particular needs of each. Chief amongst these systems with respect to popularity and deployment is the mobile or cellular phone, and it has been estimated that there are over 4.6 billion subscriptions worldwide.
Generally, wireless communications involve a radio frequency (RF) carrier signal that is variously modulated to represent data, and the modulation, transmission, receipt, and demodulation of the signal conform to a set of standards for coordination of the same. Many different mobile communication technologies or air interfaces exist, including GSM (Global System for Mobile Communications), EDGE (Enhanced Data rates for GSM Evolution), and UMTS (Universal Mobile Telecommunications System). Various generations of these technologies exist and are deployed in phases, with one common third generation (3G) UMTS-related modality referred to as UMTS-FDD (frequency division duplexing) being W-CDMA (Wideband Code Division Multiplexing). Besides mobile communications modalities such as these, mobile phones also incorporate local area data networking modalities such as Wireless LAN, or WLAN (IEEE 80.11x).
A fundamental component of mobile handsets, or any wireless communications system for that matter, is the transceiver, that is, the combined transmitter and receiver circuitry. The transceiver encodes the data to a baseband signal and modules it with an RF carrier signal. Upon receipt, the transceiver down-converts the RF signal, demodulates the baseband signal, and decodes the data represented by the baseband signal. An antenna connected to the transmitter converts the electrical signals to electromagnetic waves, and an antenna connected to the receiver converts the electromagnetic waves back to electrical signals. Conventional mobile handset transceivers typically do not generate sufficient power or have sufficient sensitivity for reliable communications standing alone. Thus, additional conditioning of the RF signal is necessary. The circuitry between the transceiver and the antenna that provide this functionality is referred to as the front-end module, which include a power amplifier for increased transmission power, and/or a low noise amplifier for increased reception sensitivity.
A functionality frequently demanded of the mobile phone networks by its users is multitasking, particularly with regard to simultaneous use of the mobile communications or cellular network modality for voice calls, and the data communications or WLAN modality to browse the Internet or download data files. There are a number of challenges associated with this, one of these being the crowded RF environment within which the WLAN subsystem operates. Furthermore, some mobile communications subsystems such as WCDMA/UMTS utilize a frequency domain duplex protocol in which the transmitter and the receiver are always activated. The transmissions from the WLAN subsystem, notwithstanding different operating frequencies, tend to cause spurious noise and interference. The diminutive size of the handset and the attendant necessity for positioning the respective antennas of the different communications subsystems in relatively close proximity to each other add another layer of challenges. Within these restrictions, one of the important objectives is to prevent the base band and other types of noise from the WLAN communications subsystem from interfering with reliable cellular network reception, without overloading or de-sensitizing the mobile communications subsystem.
Consequently, filtering is critical in the implementation of multimode mobile communications handsets. In the combined WCDMA and WLAN operating environment, band-pass co-existence filters are used to minimize the degradation of the WCDMA receive chain sensitivity. Additional rejection of unwanted spectrum emissions such as harmonics of WLAN transmissions may be possible with such filters. However, these are typically low temperature co-fired ceramic (LTCC) devices that tend to be bulkier and thereby increase the overall footprint of the printed circuit board. Furthermore, such co-existence filters also degrade the performance of the WLAN system due to increased current consumption and degraded reception sensitivity. Recently, different types of filters such as SAW (surface acoustic wave) or BAW (bulk acoustic wave) are being utilized. The fabrication of such filters can be accomplished with a smaller footprint, but the costs are significantly higher.
Some filtering can be incorporated into the WLAN power amplifier chain in order to minimize the demand for external filtering. The filter is typically connected at the input of the power amplifier, and though it would be advantageous to fabricate the same on-die, the performance of such implementations is inadequate. Problematically, the fairly low Q-factors of on-die components necessitated the addition of an external co-existence filter. In particular, such external filters connected to the output of the WLAN chain have an insertion loss of approximately 1.5 dB to 2.0 dB. This would require a higher output from the power amplifier, thereby leading to greater current consumption from the power source (battery). The insertion loss associated with the external co-existence filter also decreases the receive sensitivity of the WLAN chain, adversely affecting the link distance and data throughput.
Thus, there is a need in the art for eliminating the external co-existence filters from front end circuits implementing combined WLAN and WCDMA wireless communications modalities while retaining adequate performance parameters for both. Consequently, there is a need for the on-die fabrication of the co-existence filter, which would reduce the cost of the front end system, reduce its size, as well as its current consumption during WLAN transmission. It would be advantageous for such front end circuit to have increased sensitivity during WLAN reception, as well as high sensitivity during WCDMA reception.